In the X-ray CT apparatus, a helical scan method and a multi-detector method are adopted widely in order to obtain tomographic images of an object three-dimensionally and precisely. For this reason, a rotation speed of a scanner becomes high, the number of rows of a detector increases and a size of data output from a detector becomes huge. Therefore, it is important to suppress increase in the cost for data communications and data storages in an X-ray CT apparatus.
Accordingly, a method of compressing original data, output from the rotational part of the gantry in the detector side, in the DAS (Data acquisition system) is suggested for reduction of the data size to be transmitted in the X-ray CT apparatus. It is mainly between the rotational part and an irrotational part in the gantry and between the irrotational part of the gantry and the data processing system, for various correction processing and image reconstruction processing, that data transmission is performed in the X-ray CT apparatus.
In these parts with data transmission, it is necessary to guarantee the transmission rates in order to avoid a serious failure of data loss. The cost for a data transmission depends on the transmission rate to be guaranteed. Therefore, in order to suppress the increase in transmission cost by information compression, it is necessary to compress data by an information compression method which can secure a predetermined compression ratio not so as to exceed the transmission rate guaranteed for the data transmission system.
By a reversible information compression method, a compression ratio cannot be guaranteed theoretically although the information included in data is reservable. For example, truly random data cannot be compressed at all by a reversible information compression method. Therefore, a reversible information compression method cannot guarantee a transmission rate smaller than that in the case of transmitting the data before the information compression. In other words, the data transmission portion must guarantee the transmission rate in the case of transmitting the data before the compression. For this reason, the transmission cost cannot be reduced if a reversible information compression method is used.
On the other hand, if data is compressed by the irreversible information compression method, the transmission rate can be guaranteed by adjustment of the compression ratio. Therefore, it is necessary to use an irreversible compression method as the information compression method in order to guarantee a transmission rate.